If you are not breastfeeding, your fertility will return quickly. Moms who breastfeed exclusively & ecologically (no bottles, pacifiers, supplementation of any kind, no solids, no water) can't really get pregnant in the first 3 months postpartum. For more info: http://www.kellymom.com/bf/normal/fertility.html
It is possible to get an early return of fertility if breastfeeding exclusively though I've never met any exclusively breastfeeding mom who got pregnant within the first 6 weeks postpartum.
Anyway, I'd imagine that you could take a pregnancy test whenever you want! And from there, you could get a range of your EDD. Wow, I can't imagine being pregnant again so soon after 9 months of pregnancy/giving birth - if you are pregnant again, please make sure you eat *really* well/healthfully since your body has been through a very taxing 9 months already and is facing another 9 months of growing *another* baby. Best of luck to you!